Ultrasonic imaging system and method wtih focusing correction

ABSTRACT

Ultrasonic imaging apparatus and method are shown which include electronic correction of focus defects produced by acoustic refractive index inhomogeneities within an object (14) being imaged. The region of interest (38) within which focus correction takes place is selected by the operator using control (42 or 84). The imaging system includes adjustable time delays (28-l through 28-c) through which return signals from transducers (10-l through 10-n) pass. Outputs from the delays are summed (30) and the resultant signal is envelope detected (32). The envelope detector output is prepared for display at display (36) by scan converter (34). The output from a focus correction delay control circuit (64) is used to control delay times of individual delays (28-l through 28-c) to provide for a delay profile across operative elements of transducer array (10), which delay profile includes delay profile components that correspond to low order terms of a series expansion, such as a Fourier series (FIG. 4). The adjustable signal delays (28-l through 28-c) are simultaneously adjusted by the focus correction delay control (64) during selection of delay profile component amplitudes which reduce focus defects within the region of interest 38. Either manual (FIG. 1) or automatic (FIG. 5) focus correction is provided.

FIELD OF THE INVENTION

This invention relates generally to ultrasonic imaging method and meanswith focusing correction for inhomogeneous media.

BACKGROUND OF THE INVENTION

Ultrasonic images made in a media that is inhomogeneous in the acousticrefractive index, i.e. in its speed of sound, generally exhibit focusingerrors. For imaging systems utilizing multi-element transducer arraysand adjustable time delay means connected to operative elements of thearray for focusing and beam steering, it is possible to correct suchfocus errors to some degree by imposing adjustments to the time delays.

One prior art method for accomplishing reduction of focus defectsincludes cross correlating short segments of the signals received byadjacent or nearby array elements, corresponding to a point in theobject space for which the correction is to be made. Thecross-correlation coefficient should be maximum at a time shift equal tothe beam steering and focusing delay required to image at that point. Ifit differs, a time shift correction is introduced to bring it intoconformity. All time-shifted corrected waves are summed.Cross-correlation methods for correcting for distorting medium are shownin U.S. Pat. No. 4,484,477 by Buxton, U.S. Pat. No. 4,471,785 by Wilsonet.al., U.S. Pat. No. 4,817,614 by Hassler et al and in an article"Phase aberration Correction Using Signals from Point Reflectors andDiffuse Scatterers: Basic Principles" Flax et.al., IEEE Trans. onUltrasonics, Ferroelectrics, and Frequency Control, vol. UFFC 37, No. 5,September 1990, pp. 758-767.

Another prior art method for at least partially correcting for focuserrors produced by inhomogeneities in the media includes arbitrarilytime shifting signals from each element or small group of elements andtesting to see if the effect was to increase or decrease the quality ofthe detected summed signal corresponding to the region in the objectspace in which focus is to be improved. Each element or small group ofelements is corrected in turn until the process converges. Quality ofdetected summed signal in the region of interest (ROI) may be determinedby measuring the "speckle" amplitude, i.e. the mean of theRayleigh-distributed magnitude, within the region of interest. Examplesof such methods are shown in the following articles: "Phase AberrationCorrection in Medical Ultrasound Using Speckle Brightness as a QualityFactor," Nock et al, J.Acoust. Soc. Am. 85(5), May 1989, pp. 1819-1833and "Experimental Results With a Real-Time Ultrasonic Imaging System forViewing Through Distorting Media," Trahey et al, IEEE Trans. onUltrasonics, Ferroelectrics, and Frequency Control, Vol. UFFC-37, No. 5,September 1990, pp. 418-427.

A typical ultrasonic imaging apparatus may include 64 or 128 transducerelements and associated adjustable signal delay means. Where delays areadjusted individually, or in small groups, when correcting for focusdefects produced by velocity inhomogeneity of the object underexamination, as in the prior art, a relatively large amount of time isrequired to make the corrections. Whenever the region of interestchanges, or the transducer array is moved relative to the object, thetime-consuming correction process must be repeated.

SUMMARY AND OBJECTS OF THE INVENTION

An object of this invention is the provision of an improved ultrasonicimaging method and apparatus which includes means for rapidly reducingor correcting for focusing defects produced by acoustic refractive indexinhomogeneities within a region of interest within a section beingimaged.

An object of this invention is the provision of an improved ultrasonicimaging method and apparatus of the above-mentioned type whereinreducing or correcting for focusing defects may be automaticallyperformed.

An object of this invention is the provision of an improved ultrasonicimaging method and apparatus of the above-mentioned type which includesmanual correction control.

In accordance with the present invention a section within aninhomogeneous object is insonified using a transducer array whichincludes an array of transducer elements such as a linear, annular,two-dimensional array, or the like. Ultrasonic waves received by thetransducer array are converted to electrical signals which are suppliedto an array of adjustable signal delay means for electronic focusing,steering and/or stepping as required, and for correction of focusdefects produced by acoustic refractive index inhomogeneities within aregion of interest within the section. Outputs from the signal delaymeans are summed, and the resultant signal is envelope detected. Scanconverter means converts the envelope detected signal for display atvisual display means. The adjustable signal delay means are controlledto provide for a composite delay profile across operative elements ofthe transducer array, which composite delay profile comprises delayprofile components corresponding to low order terms of a seriesexpansion, such as a Fourier or power series. The amplitudes ofindividual delay profile components are controlled by control of theadjustable signal delay means for selection of delay profile componentamplitudes that reduce focus defects within a selected region ofinterest within the section being imaged. Adjustable signal delay meansfor operative transducer elements are substantially simultaneouslyadjusted when controlling the amplitude of individual delay profilecomponents. With a change in delay profile component amplitude, theimage quality within the region of interest is observed, and theamplitude which results in the best quality is selected for use. Qualityof image within the region of interest may be determined automaticallyor by observation by the operator. For automatic operation the meanspeckle amplitude, or brightness, of the image within the region ofinterest may be determined to provide a measure of quality. When theamplitude of the individual delay profile components have been adjustedfor maximum reduction of focus defects the resultant composite delaycorrection profile is employed during imaging operation. When adifferent region of interest is selected, the above-described process ofestablishing a delay correction profile for the selected region ofinterest is repeated.

The invention, together with other objects, advantages and features willbe more fully understood from a consideration of the following detaileddescription of certain embodiments thereof taken in connection with theaccompanying drawings. It here will be understood that the drawings arefor purposes of illustration only, the invention not being limited tothe specific embodiments disclosed therein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference characters refer to the sameparts in the several views:

FIG. 1 is a block diagram showing an ultrasonic imaging system withmanual focusing correction embodying the present invention;

FIG. 2 shows a display with a region of interest (ROI) within whichfocusing defects are reduced by use of the present invention;

FIG. 3 diagrammatically shows a linear transducer array together withtransmitted wave fronts distorted by inhomogeneities in the acousticrefractive index of the acoustic medium;

FIG. 4 shows a plurality of delay profiles corresponding to low orderspatial frequency terms of a Fourier series;

FIG. 5 is a block diagram which is similar to FIG. 1 but showingautomatic focusing correction;

FIG. 6 is a flow diagram for use in explaining operation of the focusingcorrection system of FIG. 5; and

FIG. 7 shows a control panel for use in an ultrasonic imaging systemthat includes both manual and automatic means for focusing correction.

The present invention is based upon the observation that some patternsof focusing errors are more likely than others. In particular, if adesired ensemble of phase shifts for focusing correction were to bedecomposed into an infinite series, such as a Fourier series, themajority of the correction would be contained in the lower order, i.e.the low spatial frequency, terms. It was recognized, therefore, that itis possible to produce a high degree of correction by adjusting only afew coefficients of the expansion terms. In accordance with the presentinvention, delays are adjusted simultaneously during adjustment of thelow spatial frequency terms which avoids having to adjust all of thedelays independently, or in small groups, as in prior art arrangements.

Reference now is made to FIG. 1 wherein there is shown one embodiment ofthis invention comprising a linear transducer array 10 which includestransducer elements 10-l through 10-n where, in practice, n often equals64 or 128. Preferably, the array is provided with a cylindrical lensfocusing means, not shown, for beam focusing in a plane normal to theplane of the section 12 within the object 14 to be imaged. With theillustrated arrangement, the section 12 lies in the longitudinal planeof the transducer array 10 for B-scan imaging. As will become apparent,the focus correcting system is not limited to use with the illustratedB-scan imaging system. For example, focusing correction in a C-scanimaging system also is possible.

Transducer array 10 is included in a pulsed ultrasonic B-scan imagingsystem which also includes a transmitter comprising pulsers 16 which aresupplied with recurrent timing pulses from a timing and control unit 18for on-off control thereof. With a pulser turned on, a high frequencyultrasonic energy pulse is generated which is connected throughtransmit-receive switch unit 20, comprising T/R switches 20-l through20-c, to switching matrix 22 and thence to a selected transducerelement. Timing and control signals from unit 18 are supplied to theswitching matrix for selecting a group of transducer elements to beactivated during pulse transmitting and receiving operation. Focusing ofthe transmitted beam is provided by timing of the operation of pulsers16. The axis of the transmitted beam is shifted along the transducerarray dependent upon the set of adjacent transducer elements employedduring the transmitting operation which, in turn, is dependent upon theoperation of switching matrix 22.

Reflected ultrasonic waves from discontinuities within thepulse-insonified object 14 received by the transducer array areconverted to equivalent electrical signals by individual transducerelements thereof. Received signals pass through switching matrix 22 andtransmit-receive switches 20 to an amplifier array 24 which includesamplifiers 24-l through 24-c. Signals from the amplifiers are convertedto digital form by analog to digital converters 26-l through 26-cincluded in A/D converter array 26. The A/D converter outputs areconnected to an array 28 of digital delay means 28-l through 28-c, whichdelay means may be implemented as dual port addressable memories of atype illustrated in U.S. Pat. No. 4,484,477. As described in detailhereinbelow, delay means 28-l through 28-c are employed during thereceiving operation for both focusing of the transducer array and forcorrecting for focus defects due to inhomogeneities in object 14. Ifsector scanning is employed, delay array 28 could also be employed toprovide for such sector scanning.

Outputs from the delay means 28-l through 28-c are supplied to a summingcircuit 30, and the summing circuit output is connected to an envelopedetector 32. The envelope detector may be implemented by a full waverectifier operation followed by low pass filtering. The output fromenvelope detector 32 is connected to a scan converter 34, and the scanconverter output is, in turn, connected to display unit 36 for visualdisplay of an ultrasonic image. In FIG. 2, an example of a display 37provided by the imaging system is shown which may include tick marksalong sides thereof for use in distance measurements. When focuscorrection is turned on, an operator-selected region of interest (ROI)38 also is displayed as shown in FIG. 2, within which region focuscorrection is to be provided. In FIG. 1, the selected region of interest38A within section 12 to be imaged is shown. A control panel 40 is shownin FIG. 1 which includes ROI control means 42 for selecting the regionof interest within the display and the size thereof when focuscorrection control switch 44 is turned on.

In FIG. 3, to which reference now is made, a transducer array 46 isshown together with received waves 48 which are distorted as a result ofan inhomogeneous media within which the waves are propagated. To correctfor the resultant focus defect, time delays must be imposed on thesignals from each transducer element, either during transmission orreception or both. The time delay profile across elements of the arraywhich provides for total correction is identified by reference numeral50 shown in solid line, with delay time being shown along time axis, t.

As noted above, in accordance with this invention, a correction termmade up of low order terms of a series expansion, such as a Fourierseries, is employed in the focus correction process. Using only severalof the lower order terms, a delay profile 52 such as shown in brokenline may be obtained to provide good focus correction.

Preferably, a series which minimizes the higher order coefficients fortypical object-dependent error regimes is chosen for use with thisinvention. Two suitable series include the Fourier Series and the PowerSeries. ##EQU1## -N/2≦n≦N/2, for an N-element array, where:

n is the transducer element number,

a and b are constants,

and,

m is an integer. ##EQU2## -N/2≦n≦N/2, for an N-element array where:

d and e are constants.

Elements of the array refer to operative elements where not alltransducer elements are employed every transmission/receiving operationof the ultrasonic imaging system. In the illustrated system of FIG. 1,different groups of transducer elements, selected by switching matrix22, are employed for scanning of the beam. For sector scanningoperation, all transducer elements may be employed, with scanning of thebeam being provided by timing of operation of pulsers 16 duringtransmission, and steering adjustment of delays 28 during reception.

Delay profile components corresponding to low order spatial frequencyterms of a Fourier series are shown in FIG. 4, to which figure referencenow is made. There, operative elements 10' of transducer array 10 areshown together with a plurality of different delay profilescorresponding to different low order terms, and different delay profileamplitudes. For simplicity, the profiles are shown as lines rather thana series of points at intersections of horizontal lines extending fromindividual transducers and said lines. For purposes of illustration,three frequency terms for both the sine and cosine functions thereof,together with zero amplitude and four different non-zero amplitudes, areshown. As will become apparent hereinbelow, delays provided byadjustable signal delay means included in delay array 28 are adjusted bysumming those delay profile components, and profile componentamplitudes, which provide maximum correction of focus defects.

Returning to FIG. 1, control signals for controlling delays provided bydelay means 28-l through 28-c are obtained from summing circuits 60-lthrough 60-c, respectively. Focusing during the receiving operation isprovided by focus delay control means 62 having outputs connected tosumming circuits 60-a through 60-c. Focus correction delay signals alsoare supplied to summing circuits 60-l through 60-c from focus-correctiondelay control means 64 for summing with the focus delay control signals.The summed focus delay and focus correction delay control signals aresupplied to delay means 28-l through 28-c for simultaneous focus andfocus correction control during receiving operation. Where beam steeringis employed, as shown in FIG. 5, summing circuits 60-l through 60-c alsoare supplied with beam steering delay signals for simultaneous steering,focusing, and focus correction control.

For manual focus correction, focus correction switch 44 is turned on,and ROI control 42 is adjusted for selection of a region of interest 38at display 36. The first low order spatial frequency term to be testedis selected by setting of frequency switch 66 and sin-cos switch 68. Inthe illustrated switch positions the sine of the first frequency term,the lowest spatial frequency term, is selected which, in FIG. 4, isidentified by the sin2πn/N family of curves. Then, while watching theselected region of interest at the display, the profile amplitude switch70 is switched to a position which provides the best quality of displaywithin the region of interest as determined by operator observation.

Together, switches 66, 68 and 70 control a delay profile generator 72having signal outputs corresponding to the selected spatial frequencyterm, selected by switches 66 and 68, and the selected amplitudethereof, selected by switch 70. Delay profile signals from delay profilegenerator 72 are momentarily stored in a correction memory 74, thecontents of which memory are read out to focus correction delay controlunit 64 which, as described above, controls delays 28 for correctionfocus defects.

When the sin2πn/N amplitude value which provides for greatestimprovement in focusing within the region of interest is determined,store profile switch 76 is momentarily actuated for storage of theselected delay profile component from delay profile generator incomputer memory 74. With the first term stored in memory 74, sin-cosswitch 68 is moved to the cosine position for selection of the cos2πn/Nterm. Delay profile generator 72 now generates one of the cos2πn/N delayprofiles illustrated in FIG. 4. Again, while observing display 36, theamplitude of the delay profile is adjusted by use of profile amplitudeswitch 70 to a position which provides for best focusing of the displaywithin the selected region of interest. Momentary actuation of storeprofile switch 76 stores the selected cos2πn/N profile term incorrection memory 74. The previously stored sin2πn/N profile term storedin memory 74 is combined at focus correction delay control unit 64 withthe cos2πn/N profile term such that both spatial frequency termscontribute to focus correction.

Switch 66 then is switched to the second frequency, and the sin2π2n/Nand cos2π2n/N profile terms are tested in the manner described above forselection of profile amplitudes which result in the most focused displaywithin the region of interest. The process may be repeated for the thirdand any higher frequency terms of the series expansion to betterapproximate the best achievable correction. As noted above, the selectedspatial frequency terms with selected amplitudes stored in correctionmemory 74 are combined at focus correction delay control unit 64 for usein establishing a resultant delay correction profile (such as profile 52shown in FIG. 3) that provides for best focusing within the region ofinterest. Since signal delays are simultaneously adjusted during theselection of frequency terms, and amplitudes thereof, the process ofselecting delay profiles that result in reducing focusing defectsrequires a relatively small amount of time. The operator is free tochange the region of interest as often as desired knowing that aninordinate amount of time will not be required to correct for focusingdefects each time the region of interest is changed.

Obviously, the invention is not limited to manual control of focusdefect correction. In FIG. 5, to which reference now is made, anultrasonic imaging system employing automatic, machine-implemented,means for reducing focus defects is shown. The imaging portion of thesystem, apart from focus correction, may be of substantially the sametype shown in FIG. 1 and described above. For purposes of illustration,a system employing sector scanning is shown in FIG. 5, wherein a sector12A within object 14 to be imaged is shown. Sector scanning is providedby use of beam steering delay control unit 78, outputs from which aresupplied to summing circuits 60-l-60-c along with outputs from focusdelay control unit 62 and focus correction delay control unit 64. Sectorscanning is a well known technique in the ultrasonic imaging art andrequires no detailed description.

The automatic system for reducing focus defects includes a control panel80 having a correction on-off switch 82 for enabling focus correctionoperation and a region of interest control 84 for operator selection ofthe region 38B within the image at which focus correction is to takeplace. Outputs from these controls are supplied to timing and controlunit 86, outputs from which unit provides system timing and controlsignals.

When focus correction is enabled, the output from envelope detector 32also is supplied to image memory 88 where an image of the selectedregion of interest is stored. The ROI image contained in memory 88 issupplied to ROI image quality factor detector 90 where a measure of thequality of the image at the region of interest is obtained. Initially,the delay profile generator 72 outputs no delay profile (equivalent tothe sin2πx, zero (0) amplitude, profile illustrated in FIG. 4) wherebythe output from quality factor detector 90 is dependent upon theuncorrected image at the region of interest. Means for obtaining ameasure of quality of an image are well known and include measuring the"speckle" amplitude, i.e. the mean of the Rayleigh-distributedmagnitude, within the region of interest. The use of speckle amplitude,or brightness, as a quality factor is described in publicationsincluding the article entitled, "Phase aberration correction in medicalultrasound using speckle brightness as a quality factor," by Nock andTrahey, J. Acoust. Soc. Am. 85(5), May 1989, pp. 1819-1833, the entirecontents of which article is incorporated by reference herein. Theresultant quality factor is stored at quality factor storage unit 92,the output from which storage unit is supplied to comparator 94.

At quality factor comparator 94, a comparison of the current qualityfactor with quality factors obtained earlier using different profileamplitudes is made. If no prior quality factors are included at storage92, no comparison takes place, and the current quality factor remains instorage 92, together with the identity of the profile term and amplitudeused to obtain the quality factor. The delay profile generator 72 thenis switched to change the amplitude of the selected delay profile term.For example, the profile amplitude may be switched from profile 0 toprofile +1 of the sin2πn/N term shown in FIG. 4. The image within theregion of interest resulting from use of the newly selected delayprofile amplitude is stored at ROI image storage 88, and the qualitythereof is determined at image quality factor detector 90. The resultantcurrent quality factor is stored at storage 92. Now, the current qualityfactor, using, e.g. the sin2πn/N, amplitude 1 profile as shown in FIG.4, is compared at comparator 92 with the earlier-obtained qualityfactor, using, e.g. the sin2πn/N, amplitude 0 profile, and the qualityfactor that is best, together with the profile amplitude identifying thesame, is stored at storage 92.

The process of adjusting delay profile amplitude, calculating theresultant quality factor, and storing the identity of the delay profileamplitude that provides for the best quality image within the region ofinterest is repeated for a reasonable set of amplitudes of the selectedterm of the series expansion. When all delay profile amplitudes for theselected series expansion term have been examined, the one providing forthe best image is stored in correction memory 74. The above-describedprocess is repeated for each term of the series expansion until theamplitude of the highest useful term has been determined. At the end ofthe focus correction process, correction memory 74 contains a compositedelay profile comprising the sum of all of the low order spatialfrequency terms of the series expansion that provide for improved imagefocusing within the region of interest, which composite delay profilethen is used for focus correction.

The automatic focus correction scheme illustrated in FIG. 5 anddescribed above may be implemented using a digital computer, theoperation of which is included in the flow diagram of FIG. 6, to whichfigure reference now is made. After start step 100, the region ofinterest within which focusing correction is desired is selected at step102 under operator control of ROI control means 84. At step 104, a Pcounter is set equal to 1, and a T counter is set equal to the totalnumber of delay profile terms included in the series expansion. Thedelay profile expansion term P is selected at step 106. With the Pcounter set at 1, the first term, here the sin2πn/N term, first isselected. At step 108, the amplitude of the delay profile is adjusted,and the quality factor, QF, within the region of interest is calculatedat step 110. Next, at decision step 112, it is determined whether allamplitude levels for the selected expansion term, P, have been checked.If the decision is negative, operation returns to step 108 where thedelay profile amplitude is changed, after which the quality factor usingthe new delay profile amplitude is determined at step 110, and operationreturns to decision step 112.

When decision step 112 is affirmative, after checking all amplitudes ofthe selected expansion term, step 114 is entered where the delay profileexpansion term amplitude that provides the best quality factor isselected for use. The selected delay profile component is employedduring subsequent imaging operation, including operation during whichadditional delay profile components are being selected. Followingselection of the delay profile component at step 114, step 116 isentered where the P counter is incremented by one (1) whereupon decisionstep 118 is entered for determination of whether the P counter exceedsthe total number of terms T of the series expansion employed in thecorrection process. If not, step 106 is reentered where the next delayprofile expansion term, here the cos2πn/N term, is selected for testing.Operation continues until all expansion terms, and associated expansionterm amplitudes, have been checked for those that result in the bestquality factor measurement within the selected region of interest. Whendecision step 118 is affirmative, checking of all expansion terms iscompleted, and those selected as providing the best quality factor arecontinued to be employed in the imaging process as indicated at step120. The process of selecting delay profile component ends at step 122.

Reference now is made to FIG. 7 of the drawings wherein a control panelfor use in a modified form of this invention is shown. With theillustrated arrangement either manual or automatic focusing correctionmay be selected under control of off, on, auto and manual switches 130.With either focusing correction operation, a region of interest isselected under control of a first trackball 132. With focusingcorrection turned on, and the region of interest selected, manual orautomatic operation is selected by the operator. Under automaticcontrol, operation proceeds in the manner described above with referenceto FIGS. 5 and 6 of the drawings. Under manual operation, the spatialfrequency term, here frequency number, m, is selected under operatorcontrol of up and down switches 134 and 136, respectively. Profileamplitudes for both the sine and cosine components of the selectedspatial frequency term are simultaneously adjustable by use of a secondtrackball 138, rotation of which in a vertical direction controls theamplitude of the sine component and rotation in a horizontal directioncontrols the amplitude of the cosine component. Substantiallycontinuous, rather than step, adjustment of the profile amplitudes isprovided with this arrangement. As with other embodiments, adjustablesignal delay means for operative transducer elements are substantiallysimultaneously adjusted during operation of trackball 138 so that aminimum of time is required when adjusting for the best quality of imagewithin the selected region of interest. For manual control, operatorobservation is used to measure the image quality. When the profileamplitudes for the selected spatial frequency term have been adjusted,the selected values are stored when switching to a higher or lowerspatial frequency term under control of switches 134 and 136. The numberof spatial frequency terms for focusing correction during manualoperation is under operator control since the operator may stopselecting new terms for amplitude adjustment whenever desired. Theresultant delay profile for focus correction within the region ofinterest is made up of those delay profile components employed by theoperator in the focus correction process.

The invention having been described in detail in accordance withrequirements of the Patent Statutes, various other changes andmodifications will suggest themselves to those skilled in this art. Forexample, if, when adjusting the amplitude of a selected expansion term,the quality factor of the region of interest decreases, it may not berequired to continue to change the amplitude in the same direction.Also, the number of series expansion terms used in the focus correctionis not limited to the six terms illustrated in FIG. 4, and neither arethe amplitude selections limited to those illustrated in FIG. 4. Ifdesired, small amplitude steps may be provided for substantiallycontinuous adjustment of amplitude as described above with reference toFIG. 7. Also, as noted above, focus correction may be applied duringtransmission, rather than reception by control of timing of pulsers 16,or during both transmission and reception. Additionally, the focuscorrection method of this invention is not limited to use with B-scanoperation. For example, it also may be used with C-scan and otherimaging processes. Additionally, the novel method may be used withimaging systems which employ two-dimensional transducer arrays inaddition to the illustrated linear transducer array. Furthermore, if theentire image is of a sufficiently small area, the region of interest mayencompass the entire image. That is, the entire image and region ofinterest may be coincident. Obviously, analog delay lines may beemployed in the analog portion of the imaging apparatus in place of theillustrated digital delays. Also, computational execution of the novelfocusing correction method of this invention may be employed usingdigital computing means in an imaging system employing no discrete delaymeans. It is intended that the above and other such changes andmodifications shall fall within the spirit and scope of this inventionas defined in the appended claims.

I claim:
 1. In an ultrasonic imaging apparatus for imaging a sectionwithin an inhomogeneous object, which apparatus includes a transducerarray and transmitter means for energizing elements of the transducerarray for beaming ultrasonic energy into the section, said transducerarray receiving ultrasonic waves and converting the same to electricalsignals,an array of adjustable signal delay means to which electricalsignals from operative elements of the transducer array are connectedfor electronic correction of focus defects produced by acousticrefractive index inhomogeneities within the object, visual displaymeans, means for processing of signals from said signal delay means forvisual display of the section at said visual display means, means underoperator control for selecting a region of interest within the section,manually operated control means under operator control for controllingthe adjustable signal delay means to provide a composite delay profileacross operative elements of the transducer array, which composite delayprofile comprises at least first and second delay profile componentscorresponding to predetermined first and second spatial frequency terms,respectively, of a series expansion, time delays provided by the arrayof adjustable signal delay means being substantially simultaneouslyadjusted during manual operation of said manually operated control meansfor amplitude control of said first and second spatial frequency terms,the amplitudes of said first and second spatial frequency terms beingadjusted by operator control of said manually operated control means toprovide for a composite delay profile which reduces focus defects withinthe selected region of interest within the section.
 2. In an ultrasonicimaging apparatus as defined in claim 1 wherein the series expansioncomprises a Fourier series.
 3. In an ultrasonic imaging apparatus asdefined in claim 2 wherein said first and second delay profilecomponents correspond to sine and cosine spatial frequency terms,respectively, of the Fourier series.
 4. In an ultrasonic imagingapparatus as defined in claim 1 wherein the series expansion comprises apower series.
 5. In an ultrasonic imaging apparatus as defined in claim1 wherein said manually operated control means comprises a manuallymovable means movable in one direction for amplitude control of saidfirst spatial frequency term and movable along a second direction normalto said one direction for amplitude control of said second spatialfrequency term.
 6. In an ultrasonic imaging apparatus as defined inclaim 5 wherein said manually movable means comprises a trackball.
 7. Ina pulsed ultrasonic imaging method for imaging a section within aninhomogeneous object from reflections from discontinuities within thesection of transmitted ultrasonic energy waves, comprisinga) receivingby means of a transducer array reflected ultrasonic energy waves andconverting the same to electrical signals, b) supplying electricalsignals from the transducer array to an array of adjustable signal delaymeans having delays which are substantially simultaneously adjustedduring operation of manually operated control means, c) processingsignals from the array of adjustable signal delay means and supplyingthe processed signals to visual display means for visual display of animage of the section, d) using said manually operated control meansmanually adjusting the length of delay of the array of adjustable signaldelay means so that signals from the delay means are provided with acomposite delay profile comprising at least first and second delayprofile components that extend across operative elements of thetransducer array, said first and second delay profile componentscorresponding to predetermined first and second spatial frequency terms,respectively, of a series expansion, the amplitude of the first andsecond spatial frequency terms being adjusted by operator control ofsaid manually operated control means, e) obtaining a measure of qualityof the image within a region of interest within the section imagedduring step d), f) selecting for use during subsequent imagingamplitudes of said first and second spatial frequency terms whichprovide the best measure of quality of visual display within the regionof interest.
 8. In a method as defined in claim 7 wherein said manuallyoperated control means is movable in one direction and in a directionnormal to said one direction for amplitude control of said first andsecond spatial frequency terms, respectively.
 9. In a method as definedin claim 8 wherein said manually operated control means comprises atrackball.
 10. In a method as defined in claim 7 wherein step e)comprises visually judging the quality of image within the region ofinterest provided by the visual display.
 11. In a method as defined inclaim 7 wherein said first and second spatial frequency terms compriserespective sine and cosine terms of a Fourier series.
 12. In a method asdefined in claim 7 wherein the first and second delay profile componentscorrespond to low order spatial frequency terms of the series expansion.13. In a method as defined in claim 7 wherein said series expansioncomprises a power series.